1. Field of the Invention
The present invention relates to a charged particle beam drawing apparatus, and a method of manufacturing an article.
2. Description of the Related Art
To miniaturize a semiconductor circuit, a multibeam drawing apparatus must form an ultrafine pattern, so an electron beam is focused on a substrate to a diameter of about 10 nm. To do this, the final electrostatic lens which focuses an electron beam must have a focal length as short as, for example, about 1 mm or less. Since the electrostatic lens is formed by three electrode plates, the distance between a resist and the electrode plate closest to the resist is 0.5 mm or less when each electrode plate has a thickness of 0.2 mm. This electrostatic lens is arranged in a one-to-one correspondence with the electron beam. To efficiently draw a pattern, the multibeam drawing apparatus uses several tens of thousands of electron beams. To form images of these several tens of thousands of electron beams, the electrostatic lens is formed by electrode plates, including holes through which electron beams equal in number to the holes pass. These large numbers of electron beams are guided at an interval of, for example, 0.05 mm. Hence, if a total of 250 thousand electron beams are used, electrostatic lenses are arranged in a number corresponding to the 500×500 beams. The electrostatic lenses must have an area of 25 mm×25 mm (by simple arithmetic). However, to individually control the 250 thousand electron beams, the region to be irradiated with them is divided into blocks, each of which having an area of, for example, 5 mm×5 mm for the sake of convenience, e.g. for wiring. In one block, 100×100 electrostatic lenses are arranged. Although the electron beams can be divided by various methods, they are often guided at an interval of, for example, 5 mm, so as to efficiently draw a pattern. As a result, the electron beams are distributed into 25 blocks arranged in an area of 45 mm×45 mm. This makes it necessary to use, for example, a frame which holds these electrode plates, so the surface of an electron beam lens barrel that is opposed to the wafer, has a size of at least about 50 mm×50 mm. The wafer to be exposed has a diameter of about 300 mm, so the wafer and the electrode plate are opposed to each other in a wide area at an interval of 0.2 mm.
FIG. 7 shows a perspective view of one block of electrostatic lenses in the prior art. Referring to FIG. 7, 10,000 openings 1, which form lenses for focusing 100×100 beams, are formed in a first region 2, through which charged particle beams pass and which are indicated by a gray portion. FIG. 7 shows on the upper left side a sectional enlarged view of an elliptical portion in the lower view of FIG. 7. As shown in the enlarged view of FIG. 7, three electrode plates 3a, 3b, and 3c in which openings 1, each of which has a diameter of 0.05 mm and forms the electrostatic lens, are equidistantly arrayed, are arranged with spacers 6 between them. Because all the electrode plates 3a, 3b, and 3c have a thickness of about 0.2 mm, the openings 1, each with a diameter of 0.05 mm, have a cylindrical shape. The openings 1 which form the electrostatic lens are arrayed in a 100×100 matrix within the block so as to form an opening group; a second region 4 in the white background around the opening group includes no openings because no charged particle beams pass through it. When 5×5 blocks are arranged, electrostatic lenses are arranged in a number corresponding to a total of 250,000 beams.
Since a charged particle beam is scattered and also considerably attenuated by the gas components present in the atmospheric air, an electron optical system which controls a charged particle beam in a charged particle beam drawing apparatus is maintained in a vacuum so as to prevent the attenuation of the charged particle beam. The gap between the wafer and the electrode plate is similarly maintained in a vacuum. When the charged particle beam drawing apparatus starts to draw using a charged particle beam, it irradiates the resist coated on the wafer with the charged particle beam. Bonds of molecules that compose the resist in the portion irradiated with the charged particle beam temporarily break. Depending on the type of resist, the cross-linking reaction then progresses, so a difference in molecular state occurs between the position irradiated with the charged particle beam and that which is not irradiated with the charged particle beam, thereby forming a drawing pattern. When the molecular state changes upon the irradiation of the resist with the charged particle beam, the resist constituent material partially vaporizes and is outgassed from the resist.
The moment the resist is irradiated with a charged particle beam, a large amount of components mixed with organic components are outgassed from the resist. Even after completion of the irradiation of the resist with the charged particle beam, the resist continues to release outgassed components while its decomposition reaction progresses. The dwell time for which the residual gas remains is determined depending on the gas exhaust capacity of the resist periphery. The wafer and the electrode plate opposed to it are opposed at an interval of 0.2 mm in an area of, for example, about 50 mm×50 mm, as described earlier. In this state, when the resist on the wafer is irradiated with a charged particle beam, some decomposed molecules are outgassed from the resist and fill the space between the wafer and the electrode plate. It takes much time for components outgassed from the resist to pass through the gap between the wafer and the electrode plate and migrate horizontally outwards. Some components of the gas pass through each opening in the electrostatic lens and migrate to the side (upwards) opposite to the electrostatic lens when viewed from the wafer, but this amount is relatively small. Furthermore, because the irradiation with the charged particle beam continues, components outgassed from the resist remain in the space between the wafer and the electrode plate. As a result, the pressure in this space rises. Since most resists contain organic substances, organic molecules outgassed from the resist remain in this space and therefore adhere to and deposit on the surface of the electrode plate. This phenomenon is considered to happen as secondary electrons generated by the wafer surface irradiated with the charged particle beam reach the electrode plate opposed to the wafer, and act on organic molecules physically absorbed in the wafer to combine the organic molecules with the surface of the electrode plate. When the opening shapes of the electrostatic lenses change upon deposition of the organic molecules on the electrode plate of the electrostatic lenses, the focal point or focal state of the charged particle beam also changes, so the drawing accuracy degrades. Also, when the deposited organic molecules are charged by the secondary electrons, the electric field in the electrostatic lenses is disturbed, so the position of the focal point or focal state of the charged particle beam deviates. This also adversely affects the drawing accuracy.
Further, a voltage as high as several kilovolts is applied to the electrode plates of the electrostatic lenses so as to focus the charged particle beam. Therefore, when components outgassed from the resist remain, the degree of vacuum in the space in which the electrostatic lenses are arranged decreases, leading to discharge between the electrode plates. Japanese Patent No. 3728217 discloses a method of exhausting the residual gas remaining between the electrode plates of the electrostatic lenses. In the method disclosed in Japanese Patent No. 3728217, exhaust ports and valves for exhausting the gas in the spaces formed by the opposed electrode plates are arranged in a number corresponding to the number of spaces. This method is effective in exhausting the residual gas remaining between the electrode plates. However, when the interval between the electrode plates is very narrow or wide and the pressure of the residual gas or remaining gas is 1 Pa or less, it is so difficult to increase the conductance that the exhaust capacity does not always improve, so the efficacy of this method is poor.